为进一步认识煤层气井压裂裂缝起裂和扩展机制,利用真三轴大尺寸水力压裂实验系统,采用取自矿区原煤煤块制作的试样,通过真三轴压裂实验,研究了不同围压组合条件下煤层压裂裂缝的起裂和扩展特征.研究表明:地应力状态和煤层中发育的天然裂隙对压裂裂缝的起裂和扩展有着直接影响.小规模天然裂隙的存在使得裂缝扩展压力出现频繁小幅波动,压裂裂缝在扩展过程中多见路径转向,压裂过程中煤层天然裂隙的大量开启造成压裂液大量滤失直接损耗压裂时的水力载荷,不利于压裂形成高导流能力的水力裂缝.当三向主应力差较小时,煤层压裂产生的裂缝宏观形态更为复杂,可能会同时产生垂直缝和水平缝.
For further understanding of hydraulic fracture initiation and propagation mechanisms of coalbeds, hydraulic fracturing experiments under triaxial stress were carried out on the raw coal specimens from a coalmine with a large-size true triaxial hydraulic fracturing test system. The initiation and propagation characteristics of coalbed hydraulic fracture were studied under different triaxial stress states. The experimental results indicate that the stress state and coal cleat had a direct effect on hydraulic fracture initiation and propagation. The existence of small-scale cleat caused the hydraulic fracture propagation pressure to fluctuate frequently within a narrow range. The swerve of many fracture propagation paths was observed, and the filtration of fracturing fluid led to loss of propagation pressure. These problems are not conducive to the formation of high conductivity hydraulic fracture. The coalbed hydraulic fracture will be more complex with lesser degree of principle stress difference, and horizontal and vertical fractures may be both generated. This experiment may provide references for further exploration of coalbed fracturing mechanisms.
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